Fish
Brown Trout
Salmo trutta · also called sea trout, Loch Leven trout, German brown
The brown trout is the trout of European poetry, the wary fish of the chalk stream, and the reason fly fishing became an art rather than a way to catch dinner. It is also one animal wearing many disguises: a hand-sized stream fish, a 20-kilogram lake monster, and a silver sea-runner are all the same species.
Overview
Salmo trutta is native to Europe, North Africa, and western Asia, in cold rivers, lakes, and coastal seas. It is golden-brown with black and red spots, the red often haloed in blue. It belongs to the Atlantic salmon’s genus, so it is a closer relative of that fish than of the rainbow trout. Small fish eat insects; large ones turn to fish, and the biggest hunt mice and ducklings at night.
Many forms, one fish
A river “brownie”, a deep-lake giant, and the sea-run sea trout — silver, salmon-like, returning from the ocean to spawn — are all Salmo trutta, and the forms interconvert within a single river.
A worldwide invader
Shipped on ice to Tasmania, New Zealand, and North America from the 1860s, it is now on the list of the hundred worst invasive species, eating and displacing native fish and amphibians and replacing native trout as rivers warm.
Status
Least Concern as a species. Some native river and lake populations are threatened by dams, pollution, warming, and genetic swamping by stocked strains.
Brown Trout: common questions
What is the difference between a brown trout and a sea trout?
They are the same species, and the same fish can be either depending on the life it leads. A brown trout that stays in fresh water its whole life keeps the golden-brown, spotted appearance. A sea trout is a brown trout whose young migrate down to the sea, feed there for one or more years, and return to fresh water to spawn, growing larger and turning bright silver with the spots much reduced, so that it looks superficially like a small salmon. Within a single river some brown trout go to sea and become sea trout while their siblings stay and remain "brownies", and the tendency to migrate is influenced by both genes and conditions such as food supply and competition.
How do you tell a brown trout from a rainbow trout?
The surest marks are the spots. A brown trout has black and, usually, red or orange spots on a brassy-gold to olive body, and the red spots are often surrounded by a pale bluish ring; it has few or no spots on the tail. A rainbow trout has only black spots, no red, on a greener, more silvery body, a broad pink-to-red band along the flank, and heavy spotting that extends onto the tail fin. Shape and fin colour help too, but the presence of haloed red spots and a clean tail points to a brown trout, while a pink stripe and a spotted tail points to a rainbow.
Why is the brown trout considered invasive when it is a beloved sport fish?
Because outside its native Europe it behaves like any introduced top predator, eating and outcompeting the local fauna. In New Zealand and Australia brown trout have driven severe declines in native galaxiid fish and in some frogs; in parts of the western United States and the Andes they displace native cutthroat trout, char, and grayling; and across many regions they replace more sensitive native salmonids because they tolerate warmer, more degraded water. It is genuinely both — a prized quarry that anglers and fisheries agencies have spread deliberately for over 150 years, and, in most of the places they spread it, an ecological problem.
Are the big lake brown trout a different fish?
No, they are the same species living a different life. In large, deep, food-rich lakes such as those of the European Alps, parts of Scandinavia, and New Zealand's South Island, some brown trout switch almost entirely to eating other fish, grow fast, and reach great size, often called ferox trout in Britain and Ireland. Ferox were once thought to be a separate race, and some populations are genetically distinct, but they are all Salmo trutta, and the trait is a life-history strategy, becoming a piscivorous predator, that pays off where the lake can support it.